Method of treating metals by deposition of materials and furnace for implementing said method

ABSTRACT

In a physical vapor deposition method of thermochemical treatment of metals, the substance of a target which constitutes a first electrode of a treatment furnace is evaporated by ion bombardment optionally assisted by an electrical arc discharge. The particles evaporated in this manner are deposited onto a substrate at the potential of a second electrode which is different from that of the first electrode. The substrate is heated during this deposition to a treatment temperature exceeding 600° C. and preferably between 800° C. and 1,200° C. The target and its ancillary members in the furnace are continuously cooled by a flow of cooling fluid so that when the treatment temperature is reached the material of the target remains solid and the evaporation occurring at the surface of the target is effected by sublimation. The result is to improve the regularity of the deposit, its adherence to the substrate and the treatment time.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention concerns a method for the thermochemical treatmentof metals by deposition of materials onto a substrate to be treated.

It is more particularly concerned with a physical vapor deposition (PVD)method in which the substance of a target which constitutes a firstelectrode of an ion bombardment treatment furnace is evaporated and theparticles produced in this way are deposited onto a substrateconstituting a second electrode at a potential different than that ofthe first electrode.

Of course, during this process the substrate is heated to the treatmenttemperature by the heat generated by the ion bombardment or by heatingmeans provided inside the furnace, or even by a combination of these twomeans.

2. Description of the prior art

Until now the treatment temperature has been held below a particularthreshold in the order of 500° C. to avoid exceeding the temperingtemperature of the quenched steel and so to prevent deterioration of themechanical properties of the parts treated (hardness, tensile strength,etc) such as are observed in practice when this temperature is exceeded.Also, it has been necessary to eliminate the risk of deformation of thetreated parts and to remain within the temperature range allowed for thetarget and for the means used to monitor its evaporation.

This temperature limit was essential when using a magnetron to generatea magnetic field at the surface of the target to enhance evaporationwithout melting the target.

It is clear that in this case the treatment temperature produced insidethe furnace must be as far away as possible from the melting point ofthe target and that this temperature is not high enough to damage theelectromagnet circuit of the magnetron.

Furthermore, increasing the temperature above said threshold wentcounter to an advantage resulting from the use of a magnetron wherebyheating of the substrate as a result of its bombardment with high-energyelectrons from the target is reduced.

A particular object of the invention is to improve the quality of thethermochemical treatment previously described, in particular withreference to the regularity of the deposit (thickness, roughness), itsadherence to the substrate and the treatment time. Another object is tomaster the treatment of depositing metal halogenides which werepreviously difficult to use or entailed the use of highly toxicelements. It is further directed to expanding the range of depositiontreatments, in particular by enabling deposition of interdiffusedmultilayers or binary or ternary alloys and deposition followed byquenching.

SUMMARY OF THE INVENTION

To achieve these results the invention proposes to go against receivedwisdom as previously explained and to deposit the materials at atreatment temperature in a range between 800° C. and 1,200° C.

To achieve this result it proposes to use a vapor generator speciallydesigned so that the target and its ancillary members inside the furnaceare cooled at all times by a flow of cooling fluid so that when theparts to be treated are heated to the treatment temperature the materialof the target remains solid and evaporation occurs at the surface of thetarget by sublimation.

The invention is naturally concerned also with a treatment furnace forimplementing the method previously defined, this furnace comprising oneor more cooled target vapor generators installed in a vacuum furnacewith or without plasma assistance.

Because the target remains in the solid state at all times, it can bedisposed vertically or horizontally at the most appropriate location. Inpractice the vapor generator(s) must be located so that the flows ofmetal vapor produced by the various evaporators are multidirectional andproduce as homogeneous as possible a distribution of the deposit.

Embodiments of the invention will be described hereinafter by way ofnon-limiting example with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrammatic views in horizontal cross-section(FIG. 1) and in vertical cross-section (FIG. 2) of a heat treatmentfurnace fitted with four vapor generators.

FIG. 3 is a view in axial cross-section and to a larger scale of a vaporgenerator that can be used to implement the method in accordance withthe invention.

FIGS. 4 through 10 are diagrams plotting temperature as a function oftime showing different types of treatment by the method in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

The furnace shown in FIGS. 1 and 2 is a conventional heat treatmentfurnace for ion bombardment in vacuum.

It comprises a sealed enclosure 1 with a single or double metal wallspossibly in the latter case cooled by circulation of water in the spacebetween the two walls.

Inside the enclosure 1 is a casing 2 made from a thermally insulativematerial which delimits the "laboratory" 3 of the furnace.

Within this laboratory 3 the parts 4 to be treated are placed on anelectrically conductive material support 5 to which a bias voltage (forexample, a pulsed DC voltage of a few hundred volts) may be applied bymeans of a current lead-through 6 which is sealed into an opening in theenclosure and which is electrically insulated from the enclosure.

As in all vacuum heat treatment furnaces, the interior volume of theenclosure 1 communicates with an external pumping station 7 forproducing a relative vacuum (8×10⁻³ mbar to 10⁻¹ mbar, for example) anda source 8 of neutral or reactive gas.

The parts are heated to the treatment temperature, which is between 400°C. and 1,300° C. in this example, by resistive heating elements 9 madefrom graphite, for example, carried by the inside walls of the casing 2.They are cooled by circulation of gas injected into the furnace at theend of the treatment. The gas is circulated in the furnace, possiblythrough a heat exchanger, by a turbine 10. This circuit isadvantageously designed to enable pressurized gas quenching.

In this example the furnace is fitted with six vapor generators G₁through G₆, namely:

two horizontal target generators G₁, G₂ mounted on the top surface ofthe enclosure 1, and

four vertical target generators G₃, G₄, G₅, G₆ mounted on two oppositevertical walls of the enclosure 1.

Of course, each of the generators passes through the enclosure 1 and thecasing so that the target is in the laboratory near the parts 4 to betreated.

Access to the interior of the furnace is provided by a sealed andthermally insulative front door 11 which also constitutes the door ofthe casing 2.

As previously mentioned, the vapor generators G₁ through G₆, and inparticular the vertical target generators, must be designed to createthe vapor phase by direct sublimation of the metal constituting thetarget.

To this end, these generators may be designed to implement an electricalarc evaporation process which produces a metal vapor by means ofelectrical arcs moving over the surface of the target and optionallycombines the metal vapor with a given reactive gas to synthesize ametal, an alloy or a nitride, carbide or oxide type compound in the formof a thin layer on the substrate.

As shown in FIG. 3, a generator G may comprise:

a cylindrical target 12 made from the material to be evaporated and tobe deposited onto the parts 4 to be treated (it must be understood thatthe targets used in the same furnace may be of different kinds so as toproduce deposits incorporating more than one element, for example morethan one metal);

a support structure comprising a tubular sleeve 13 closed at one end 14carrying the target 12 coaxially and provided at its other end with aflange 15 inserted, fixed and sealed into an orifice in the enclosure 1of the furnace from which it is electrically insulated;

a block 16 of electrically insulative material which closes the tubularsleeve 13 at the flange 15 and through which passes an electricallyconductive bar 17 which extends coaxially inside the sleeve 13 intoelectrical contact with the target 12, the bar 17 having an outer end 18passing through the block 16 provided with means 19 for connecting it toan electrical circuit; the bar has a coaxial passage 20 at the end nearthe target 12;

a shouldered bush 21 which divides the volume between the tubular sleeve13 and the bar 17 into two chambers 22, 22';

an electromagnet coil 23 disposed around the bar 17 in the chamber 22adjacent the target 12 for creating a magnetic field near the target 12;

an insulative tube 24 which extends into the chamber 22' coaxially withthe bar 17 between the block 16 and the bush 21, this tube delimitingwith the bar 17 an intermediate volume which communicates with thepassage 20 through orifices 25 formed in the bar 17;

an arc discharge initiator device comprising an exciter electrode 27made from a refractory metal or alloy such as molybdenum, for example,parallel to the axis of the tubular sleeve 13, outside the latter, theelectrode 27 having a curved end facing towards the target 12;

a mechanism for actuating the exciter electrode 27 comprising a driveelement (actuator 28) located outside the furnace and an actuator rod 29to which the electrode 27 is fixed, the rod 29 being slidably or evenrotatably mounted in sealed bearings 30 passing through the flange 15(the seal being provided by sealing bellows 30');

a floating potential screen 31 made from graphite, for example, in theform of a ring sliding on the target 12 and carried by a tubular supportsleeve 32 coaxial with the tubular sleeve 13 and able to slide thereon;and

a cooling fluid circuit having an inlet pipe connected to an inletpassage 33 in the block 16 and discharging into the intermediate volumebetween the bar 17 and the insulative tube 24, an axial slot 34 milledinto the bar 17 and providing communication between the axial passage 20and the chamber 22, holes 35 in the bush 21 to provide communicationbetween the chambers 22 and 22', and a return pipe connected to a returnpassage 36 in the block 16.

The vapor generator previously described operates in the followingmanner:

A low-voltage (less than 100 V) high direct current (10 to 400 A)supplied by an electrical power source of the kind used in electricalarc welding is applied to the bar 17 and therefore to the target 12.

The electrode 27 causes an electrical arc discharge which is localizedon the surface of the target 12 by virtue of the floating potential onthe screen 31.

At the same time the coil 23 is energized (by conductors 23' passingthrough holes 35') to apply a magnetic field to the target 12.

This magnetic field confines the plasma and controls movement of the arcdischarge over the target 12.

The target constitutes a cathode of a plasma generator in which theenclosure 1 of the furnace or even the resistive heating elements 9constitute the anode.

The parts 4 to be treated are negatively biased during deposition by apulsed DC voltage of few hundred volts.

By virtue of the arrangements previously described, the entire generator(and in particular the target 12 and the electromagnet coil 23) iscooled by the circulation of cooling fluid. As a result the temperatureat the surface of the target can be very high, even approaching themelting point of the target material, while the remainder of thegenerator is kept at a relatively low temperature.

A three-fold process therefore occurs at the target:

abnormal luminescent electrical discharge generating a plasma on thetarget 12 because of the cathode potential and the electric field;

arc discharge caused by the electrode 27;

application of a magnetic field to the target 12 so that the intensityof ion bombardment can be increased and concentric and high-speedmovement of the arc spots can be obtained.

By reason of these three effects the surface of the target 12 isintensely heated and material is released from it by sublimation as aresult of the impact of the ions of the ionized gas and of the arcdischarge.

The material released from the target 12 is transferred into the neutralor reactive atmosphere of the furnace primarily by an ionized gas phasetransport mechanism, the effect of the electric field establishedbetween the generator and the parts to be treated depending on thedistance between them, with the possibility of giving preference to thelaterally disposed generators G₃ through G₆.

By virtue of the potential difference between the parts 4 to be treatedand the anode there occurs at the parts 4 to be treated a secondabnormal luminescent electrical discharge which improves therestructuring of the deposit of material and its adherence (by virtue ofphenomena of diffusion or of interdiffusion at the substrate/depositinterface and condensation phenomena assisted by the ion bombardment).

It is found that these phenomena are significantly accentuated in thetemperature range of the method in accordance with the invention and inparticular between 800° C. and 1,300° C.

It is naturally preferable to apply to the parts 4 a surface treatmentbefore and/or after the deposition phase, in particular to obtain asubstrate/deposit bond exhibiting a gradual change of hardness, to limitmechanical stresses in the interface area and to achieve excellentsubstrate/deposit adherence.

Various alternative pre-treatment and post-treatment processes areavailable:

a) Carbon enrichment of the surface of steel parts up to a concentrationin the order of 0.8%. This is a carburizing treatment with optionalplasma assistance.

b) Production of a surface layer over-enriched with carbon to form fineand spheroidal carbides, with a surface carbon concentration between0.8% and 2%. This is a over-carburizing treatment usually applied tosteels containing carbide-producing alloying elements (Cr, Mo, W, V, Nb,etc). Like treatment a) this treatment can be followed in the same cycleby high-temperature deposition and pressurized gas martensitic quenchingto harden the carburized sub-layer.

c) Ion nitriding titanium alloy at high temperature followed bydeposition of titanium nitride, for example.

d) It may be advantageous to carry out a high-temperature diffusion heattreatment after high-temperature deposition in order to achieveinterdiffusion between the elements of the substrate and those of thecoating.

FIGS. 4 through 9 illustrate examples of treatment by the method inaccordance with the invention.

EXAMPLE 1 (FIG. 4)

This example concerns a hybrid treatment cycle combining ion nitridingwith high-temperature deposition of titanium nitride and comprising thefollowing phases:

a first phase a₁ of heating in vacuum to 500° C.,

a homogenization phase b₁ at 500° C.,

a second heating phase c₁ from 500° C. to 850° C.,

an ion nitriding phase d₁ at 850° C.,

a heating phase e₁ from 850° C. to 900° C.,

a titanium nitride deposition phase f₁ at 900° C.,

a cooling phase g₁.

EXAMPLE 2 (FIG. 5)

This example concerns a hybrid treatment comprising ion or low-pressureover-carburizing followed by high-temperature deposition of chromiumnitride, for example with austenitization and pressurized gasmartensitic quenching. This treatment, which is suitable for Z 38 CDV 5heat treatment steels (for example, a steel for hot forging dies),comprises the following phases:

a first phase a₂ of heating in vacuum to 600° C., a period at thistemperature and then further heating to 1,000° C.,

a second phase b₂ of over-carburization at 1,000° C.,

a deposition phase c₂ at 900° C. (deposition of chromium nitride, forexample),

an austenitization phase d₂ at 1,010° C.,

a pressurized gas quench phase e₂.

EXAMPLE 3 (FIG. 6)

This example concerns a hybrid treatment combining ionover-carburization with high-temperature deposition of boron followed bya diffusion phase. This treatment is similar to that previouslydescribed except that during the phase c₃ at 900° C. boron is depositedand the next phase d₃ at 1,000° C. is a diffusion phase in which boroncarbide is formed by migration of carbon to the boron.

EXAMPLE 4 (FIG. 7)

This example concerns a hybrid treatment combining high-temperaturedeposition of chromium followed by a vacuum diffusion phase. Itcomprises:

a phase a₃ of heating in vacuum to 1,000° C. with a constant-temperatureperiod at 600° C.,

a constant-temperature phase b₃ at 1,000° C. in which the chromium isdeposited,

a vacuum diffusion phase c₃ at 980° C.,

a pressurized gas quench phase d₃.

This treatment produces a layer of high-hardness (≃2,000 HV) Cr₂₃ C₆ andCr₇ C₃ type chromium carbides.

EXAMPLE 5 (FIG. 8)

This example concerns a hybrid treatment for depositing boron similar tothe previous treatment except that the boron deposition phase b₄ and thevacuum diffusion phase c₄ are carried out at a temperature in the orderof 900° C.

This treatment produces high-hardness (2,000 HV) FeB and Fe₂ B type ironborides.

EXAMPLE 6 (FIG. 9)

This example concerns a hybrid treatment combining ion nitriding withsubsequent deposition of diffused chromium. This treatment comprises:

a phase a₅ of heating in vacuum to 580° C.,

a phase b₅ of ion nitriding at 580° C. followed by heating to 900° C.,

a chromium deposition phase c₅ at 900° C.,

a diffusion phase d₅ at 900° C. and,

pressurized gas quenching.

This treatment produces a chromium layer of very high hardness.

EXAMPLE 7 (FIG. 10)

This example concerns a carburizing or over-carburizing treatment forcarbon tool steel at high temperature by evaporating or sublimating agraphite target, possibly with a subsequent diffusion phase andpressurized gas quenching.

As shown in FIG. 10, this treatment comprises:

a phase a₆ of heating in vacuum to a temperature of 900° C. to 1,200°C., according to the nature of the steel,

an evaporation or sublimation phase b₆ with simultaneous diffusion ofcarbon to carburize the steel (with no hydrocarbon gas vector),

a pressurized gas quenching phase c₆.

Of course, the invention is not limited to these examples and many otherdeposits can be obtained. To this end the materials constituting thetarget to be evaporated may indifferently comprise:

a pure metal such as titanium, hafnium, chromium, nickel, boron ortungsten, for example,

solid carbon (high-density graphite, vitreous or pyrolytic carbon),

a binary alloy (Ti--Al, Cr--Al, Cr--Ni, Cr--Ti, Fe--Si, etc, forexample),

a multi-element complex alloy (MCrAlY, NiCoCrAlYTa, Ti--Hf--Al, etc).

Likewise, during the deposition phase a neutral gas such as argon,helium, hydrogen or another gas may be introduced into the furnace tostabilize the arc discharge, to promote ionization or to achieve aprecise working pressure. A reactive gas may be injected duringdeposition to combine with the metal vapor and form a metal nitride,oxide or carbide type compound, or a combination of these gases may beinjected to obtain hybrid carbo-nitride, oxi-nitride, etc compounds.

There is claimed:
 1. Furnace for implementing a physical vapor deposition method of thermochemical treatment of metals in which the substance of a target which constitutes a first electrode of a treatment furnace is evaporated by ion bombardment optionally assisted by an electrical arc discharge and the particles evaporated in this manner are deposited onto a substrate at the potential of a second electrode which is different from that of the first electrode, the substrate being heated during this deposition to a treatment temperature exceeding 600° C. and preferably between 800° C. and 1,200° C., and said target and its ancillary members in the furnace are continuously cooled by a flow of cooling fluid so that when the treatment temperature is reached the material of the target remains solid and the evaporation occurring at the surface of the target is effected by sublimation, said furnace comprising a sealed enclosure containing a thermally insulative material casing which delimits a laboratory in which the parts to be treated may be placed on an electrically conductive material support to which a bias voltage may be applied, the interior volume of the enclosure communicating with a vacuum pumping station and with a circuit for distribution of a neutral gas or a reactive gas that may be circulated inside the furnace by a turbine, the parts being heated by electrical heating elements inside the casing, said furnace being fitted with at least one vapor generator whose target is disposed inside the casing near the parts to be treated, said vapor generator comprising:a support structure in the form of a tubular sleeve closed at one end carrying the target and provided at its other end with a flange inserted, fixed and sealed into an orifice in the enclosure of the furnace from which it is electrically insulated; an insulative material block which closes off the tubular sleeve and through which passes an electrically conductive bar extending coaxially inside the sleeve and connected electrically to the target, said bar being connected to an electrical circuit and comprising an axial passage; an electromagnet coil around said bar near said target; and a cooling circuit comprising a cooling fluid inlet pipe passing through said block and connected to said axial passage, an orifice in said bar near said target to provide communication between said axial passage and the intermediate volume between said tubular sleeve and said bar, and a return pipe passing through said block and discharging into said intermediate volume.
 2. Furnace according to claim 1 wherein said generator further comprises an insulative tube disposed coaxially with said bar between said block and a bush and the volume between said tube and said bar communicates with said inlet pipe and with said axial passage through holes in said bar.
 3. Furnace according to claim 1 further comprising a mobile electrode adapted to initiate an arc discharge on said target.
 4. Furnace according to claim 1 wherein said generator comprises a screen at a floating potential in the form of a ring around and sliding on said target. 